Process for the surface cooling of food products

ABSTRACT

The invention concerns the surface cooling of food products. 
     Nozzles project particles of carbon dioxide snow in the direction of a conveyor. The nozzles have sufficiently fine sections (0.2 sq mm / 1.13 sq mm) to create fine particles having sizes of less than 300 microns.

The invention relates to the surface cooling of food products, of thetype in which said products are subjected, in a confined zone thermallyinsulated from the surroundings, to the cryogenic action of particlesformed by the expansion of carbon dioxide under pressure and in theliquid state through nozzles oriented toward the mean surface offered bythe products in the course of the surface cooling, and of the typeemploying spraying under pressure. This surface cooling, or crusting,permits the subsequent treatment of all food products including the mostfragile such as broccoli and mushrooms. As the case may be, after this"crusting", a deep freezing is carried out. According to the techniquerecalled above, the expansion of carbon dioxide is usually accompaniedby an intense ventilation so that it is not possible to produce thecryogenic effect of the solid particles of the carbon dioxide snowdirectly on the surface of the product to be cooled. Moreover, thecarbon dioxide snow is constituted by large particles which, inaccumulating, form a layer of snow which hardly promotes a good thermaltransfer.

An object of the present invention is to provide a process of this typewhereby it is possible to obtain remarkable performances in respect ofthe thermal transfer of the carbon dioxide snow to the surface of theproduct; further, the cooling process and the equipment are particularlysimple and reliable, with no risk of breakdown due to a clogging of theconduits by the solid particles of carbon dioxide; in addition, there isthe benefit of very high precision owing to a correct determination oftemperature in the cooling tunnel. These results are obtained, accordingto the invention, in that there is ensured a quasi-instantaneoussublimation of the solid particles of carbon dioxide in contact with theproducts by choosing nozzles of the type providing a spraying underpressure having an ejection section of between 0.2 sq mm and 1.13 sq mmand an ejection pressure of between 6 and 30 bars, producing a compositejet of fine solid particles whose sizes are between 20 microns. The finesolid particles are and 300 microns rapidly carried along in a gaseouscurrent exclusively composed of carbon dioxide resulting from saidexpansion, the flow rate and the pressure of carbon dioxide being soregulated as to produce a flow rate of fine solid particles which islower than 0.5 kg/hr per sq cm of the surface area of the product, theduration of the treatment being between 0.5 minute and 5 minutes (min).

All of these measures permit the obtainment of the mentioned results.the fineness of the particles carried along with the minimum ofventilation, i.e. with no accompanying stirring means, permit thedeposit of a fine layer of micronic particles on the products to becooled which is immediately sublimated to form a layer of snow, whichipso-facto ensures the best thermal transfer effect. Moreover, therandom dispersion of the particles outside said surfaces is avoided.Further, by an ingenious regulation of the flow rate and the pressure ofthe carbon dioxide, the surface cooling is ensured under conditions ofshort duration with a minimum flow of solid particles.

The invention also concerns a surface cooling tunnel of the typecomprising an elongated insulating body with an inlet and an outlet atthe two longitudinal ends, a flat conveyor, a plurality of nozzlesdistributed in said elongated body, connected by a conduit and valves toa tank containing carbon dioxide under pressure, and this tunnel ischaracterised in that said nozzles are all oriented toward the flatconveyor and have ejection sections of between 0.2 and 1.13 sq mm.

The invention is now described with reference to the accompanyingdrawings in which the single figure is a perspective view, with a partcut away, of a refrigeration tunnel.

With reference to the drawings, a tunnel comprises an elongatedinsulation case 1 having a rectangular parallelepipedic shape with aninlet 2 and an outlet 3 for the products to be cooled. For example, thelength of this elongated body is 3 m, its width is 0.8 m and its height1.1 m. The inlet 2 and outlet 3 have a useful width of 0.3 m and aheight of 0.2 m.

The elongated body 1 is extended beyond the inlet section 2 so as toform a support base 5 for the end of a flat conveyor 6 formed by twobelt portions 7 and 8 which pass around, at the inlet end 2 and outletend 3, rollers 9 and 10. In the example represented in the drawings, itcan thus be seen that the conveyor extends slightly beyond the inlet end2 to permit the deposit of products to be cooled, either manually orautomatically, while the outlet end of the conveyor in the region of theoutlet section 3 permits the discharge of the surface cooled products bya free fall thereof with optionally a reception of these products byanother conveyor.

The upper wall or vault 12 of the elongated case 1 acts as a support bymeans not shown for a plurality of transverse systems, such as thoseshown at 13, 14, 15, 16, 17 and 18, each equiped with a plurality ofspraying nozzles such as those shown at 21, 22, 23 and 24. These nozzlesoperate by pressure and illustratively may be those commerciallyavailable from SRAYING SYSTEM of the type TG03 or TG07. For reasons offluid flow, these transverse systems are interconnected and suppliedthrough a supply conduit 31, 32 respectively. These two conduits 31, 32are connected to a single supply conduit 35, connected through a conduit36 (equiped with a filter 52) to a source, not shown, of carbon dioxideunder a pressure of between 6 and 30 bars at the temperaturecorresponding to the liquid vapour equilibrium state of the CO at theconsidered pressure, through:

either a conduit 41 having an electrically-operated valve 42 capable ofdelivering a nominal flow rate of liquid carbon dioxide;

or through a conduit 43 having an electrically-operated valve 44 inseries with a "needle" valve 45 regulated for a low flow rate at least30% lower than the nominal flow rate, this conduit 43 being equiped onthe upstream side of the electrically-operated valve 44 with a degassingvessel 46;

or through a conduit 47 equiped, in the downstream direction, with aheater 48, a pressure-reducing valve 49, a check-valve 50 and anelectrically-operated valve 51.

In operation, the products are, as mentioned before, placed on theconveyor 6 which moves in the direction of arrow F and, in the permanentoperation mode, a temperature probe, not shown, effects the followingoperations:

when the temperature measured in the refrigeration tunnel is higher thanthe set temperature by at least a temperature deviation regulated to bebetween 0.5° C. and 15° C., the regulation is effected in such mannerthat the electrically-operated valves 42 and 45 are open, whereas theelectrically-operated valve 51 is closed. In this case, the nominal flowrate, i.e. the maximum prescribed flow rate, is transferred to thespraying nozzles 24 through the conduits 31, 32, 13, 14, 15, 16, 17 and18 and the cooling is intense;

when the temperature has dropped to such value that its deviation fromthe set temperature is relatively small (always between 0.5° C. and 15°C.), the valve 42 is closed; liquid carbon dioxide at a distinctly lowerflow rate is conveyed to the conduits 31 and 32 and reaches, as before,the nozzles 21 and 24.

The cooling effect is markedly reduced and usually the temperature ismaintained within the imposed temperature deviation. If this deviationof the temperature from the set temperature must be reduced to zero orif the temperature inside the refrigeration tunnel must be lower thansaid set temperature, the electrically-operated valve 45 is closed andthe electrically-operated valve 42 remains closed, while theelectrically-operated valve 51 is open, which produces a flow of heatedcarbon dioxide under low pressure, and therefore in the gaseous state,ensuring a permanent scavenging, even in the absence of cooling, so thatany formation of particles of solid carbon dioxide snow capable offorming an obstruction is avoided.

The spraying nozzles 21, 24 have not been described in detail, sincethey are known per se. However, they have, as mentioned before, thedouble particularity of on the one hand being all directed toward theproducts to be cooled, i.e. in a direction perpendicular to the belt 7of the conveyor 6, and on the other hand of having a relatively narrowoutlet section of between 0.2 sq mm and 1.13 sq mm, which permits boththe formation of very fine particles of carbon dioxide snow whosedimensions are between 20 and 300 microns and a directivity of theseparticles toward the products to be cooled owing to the absence of anystirring means inside the tunnel. The distance between the nozzles andthe product is on the order of 150 to 300 mm. The ejection rates of theparticles from a nozzle are:

for an ejection pressure of 15 bars, on the order of 20 to 25 m/sec at adistance of 50 mm, on the order of 8 m/sec at a distance of 250 mm;

for an ejection pressure of 18 bars, on the order of 25 to 30 m/sec at50 mm, on the order of 8 to 10 m/sec at a distance of 250 mm.

We claim:
 1. A process for surface cooling food products, of the type inwhich said products are subjected, in a confined zone thermallyinsulated from the surroundings, to the cryogenic action of particlesformed by the expansion of carbon dioxide under pressure and in theliquid state through nozzles oriented toward the mean surface offered bythe products in the course of cooling, the process comprising the stepsof subjecting the solid particles of the carbon dioxide in contact withthe products to quasi-instantaneous sublimation by spraying theparticles under pressure through nozzles having an ejection section ofbetween 0.2 sq mm and 1.13 sq mm at an ejection pressure of between 6and 30 bars, producing a composite jet from each nozzle of carbondioxide gas and very fine solid particles of sizes between 20 micronsand 300 microns, said jet being exclusively composed of carbon dioxideresulting from said expansion, maintaining the aggregate flow rate offine solid particles in all of said jets lower than 0.5 kg/hr sq cm ofsurface area of product, and continuing the spraying of said particlesby means of said jets for a period of time of between 0.5 minutes and 5minutes.
 2. A surface cooling process according to claim 1, whichfurther comprises controlling the flow of carbon dioxide under pressureand in the liquid state by the temperature measured in said confinedzone by maintaining a nominal flow rate of liquid carbon dioxide for ameasured temperature higher than a small deviation of between 0.5° C.and 15° C. from a set temperature, reducing the flow rate of liquidcarbon dioxide by at least 30% of said nominal flow rate when themeasured temperature is within said deviation, and providing a zeroliquid carbon dioxide flow for any temperature lower than the settemperature by a value at least equal to said deviation.